This invention relates generally to electrical cables, and more particularly the invention relates to power cables and methods of manufacturing same.
Electrical cables are often formed in a sheathed or coaxial arrangement with one conductor positioned within another conductor. A dielectric material provides electrical isolation between the inner and outer conductors. In low temperature cables a coolant or cryogen is provided to lower the temperature and the resistance of the cable and hence reduce the power loss within the cable. Typically, the cable is positioned within a cryogenic envelope with a liquid coolant surrounding the cable. Also, the inner conductor may be hollow with the coolant flowing therethrough.
Electrical cables operating under either ambient or cryogenic conditions require an insulator having high dielectric strength, low dielectric loss, proper mechanical properties, and reasonable cost. Heretofore for low temperature cables a number of dielectrics have been employed with varying degrees of success. One approach has used the cryogen both as a coolant and as the dielectric. This approach has been largely unsuccessful because of the poor dielectric strength of most cryogens. Another approach has been to use a vacuum for both the electric and thermal insulation of the cable. While this removes the cryogen from the dielectric region, the use of a vacuum presents certain limitations including contamination of the entire line in case of a break at any point in the cable. In yet another approach, insulative tape has been wrapped on the conductors to provide a flexible cable insulation in the presence of a cryogen. However, the breakdown voltage of taped cables is a weak function of the dielectric strength of the tape and a stronger function of the dielectric strength of the cryogen which is present in the butt-gaps of the tape.